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                        <h1 id="block">Block注意事项</h1>
<h2 id="">本小节知识点:</h2>
<ol>
<li>【掌握】Block注意事项</li>
</ol>
<hr>
<h2 id="1block">1.Block注意事项</h2>
<ul>
<li>在block内部可以访问block外部的变量</li>
</ul>
<pre><code class="lang-objc"><span class="hljs-keyword">int</span>  a = <span class="hljs-number">10</span>;
<span class="hljs-keyword">void</span> (^myBlock)() = ^{
    <span class="hljs-built_in">NSLog</span>(<span class="hljs-string">@"a = %i"</span>, a);
    }
myBlock();
输出结果: <span class="hljs-number">10</span>
</code></pre>
<ul>
<li>block内部也可以定义和block外部的同名的变量(局部变量),此时局部变量会暂时屏蔽外部</li>
</ul>
<pre><code class="lang-objc"><span class="hljs-keyword">int</span>  a = <span class="hljs-number">10</span>;
<span class="hljs-keyword">void</span> (^myBlock)() = ^{
    <span class="hljs-keyword">int</span> a = <span class="hljs-number">50</span>;
    <span class="hljs-built_in">NSLog</span>(<span class="hljs-string">@"a = %i"</span>, a);
    }
myBlock();
输出结果: <span class="hljs-number">50</span>
</code></pre>
<ul>
<li>默认情况下, Block内部不能修改外面的局部变量</li>
</ul>
<pre><code class="lang-objc"><span class="hljs-keyword">int</span> b = <span class="hljs-number">5</span>;
<span class="hljs-keyword">void</span> (^myBlock)() = ^{
    b = <span class="hljs-number">20</span>; <span class="hljs-comment">// 报错</span>
    <span class="hljs-built_in">NSLog</span>(<span class="hljs-string">@"b = %i"</span>, b);
    };
myBlock();
</code></pre>
<ul>
<li>Block内部可以修改使用__block修饰的局部变量<pre><code class="lang-objc">__block <span class="hljs-keyword">int</span> b = <span class="hljs-number">5</span>;
<span class="hljs-keyword">void</span> (^myBlock)() = ^{
  b = <span class="hljs-number">20</span>;
  <span class="hljs-built_in">NSLog</span>(<span class="hljs-string">@"b = %i"</span>, b);
  };
myBlock();
输出结果: <span class="hljs-number">20</span>
</code></pre>
</li>
</ul>

                    
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